Formation of singlet oxygen,1O2, during the O2 electroreduction reaction (ORR) on carbon electrode in Lithium bis‐(tri‐fluoromethylsulfonyl)imide (LiTFSI) dimethyl sulfoxide (DMSO) electrolyte has been detected “in‐operando” by fluorescence quenching of 9,10‐dimethylanthracene, DMA. Furthermore, the formation of 1O2 during ORR requires Li+ ions, while in the presence of the efficient physical 1O2 quencher, sodium azide, DMA fluorescence quenching is negligible. Addition of Li+ ions to superoxide solution in DMSO formed by ORR enhances DMA quenching, which is a proof that 1O2 results from superoxide bimolecular disproportionation.

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The oxygen reduction reaction (ORR) on Au electrodes has been studied in DMSO at different Li + concentrations. In-operando fluorescence decay of 9,10-dimethyl anthracene (DMA) has shown that disproportionation of lithium superoxide Li + O 2 À into Li 2 O 2 and O 2 leads to an increasing fraction of very reactive singlet oxygen ( 1 O 2 ) at high lithium concentration. Singlet oxygen has been identified as the major cause of parasitic reactions leading to capacity fading and high charge overpotential of LiÀ O 2 batteries. Rotating ring-disk electrode shows quantitative formation of soluble superoxide at low Li + concentration, a decrease in superoxide yield at high Li + concentrations is consistent with electrochemical quartz crystal microbalance (EQCM) evidence of Li 2 O 2 deposits. Differential electro chemical mass spectrometry (DEMS) confirms oxygen depletion at the electrode surface during ORR, and O 2 evolution during oxidation at 3.1 V (vs. Li/Li + in DMSO). The spurious solvent decomposition due to the very reactive 1 O 2 from superoxide disproportionation is revealed by gravimetric EQCM of insoluble by-products. Furthermore, DEMS provides evidence of CO 2 gas evolution from decomposition of Li 2 CO 3 by-product at 3.7 V (vs. Li/Li + in DMSO). Preliminary in-operando full discharge-charge tests of a LiÀ O 2 battery with 1 O 2 quencher azide resulted in stable cycling, enhanced capacity and full charge recovery in a round trip.